1. Field
This disclosure is generally related to photovoltaic systems. More specifically, this disclosure is related to a photovoltaic system that combines tunneling junction solar cells and optical concentrators.
2. Related Art
The negative environmental impact caused by the use of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.
Photovoltaic (PV) systems use solar panels to convert sunlight into electricity. A PV system includes multiple components, such as photovoltaic modules (or solar panels), frames, cables, and inverters. The cost of the PV modules contributes significantly to the cost of the entire photovoltaic system. To reduce costs, various approaches have been used to reduce the cost of each component and to improve the efficiency of the photovoltaic module. One approach is to use concentration optics that focuses sunlight to a smaller area, using a PV module that is much smaller than the size of the system. Consequently, the cost of the PV modules within the PV system can be reduced significantly. Although there are additional components, such as optical modules and a tracker, the cost of the whole system is still less than a system without the concentration optics.
There are many challenges in achieving a high-efficiency PV module with concentration optics. When the sunlight is focused to a smaller area, its intensity is greatly increased, resulting in rapid heating of the solar cells. Therefore, cooling is required. However, it is not economical to cool the temperature of the solar cells to as low as around 20° C. Instead, the solar cells will most likely operate at an elevated temperature. This is undesirable because the energy-conversion efficiency of semiconductor solar cells degrades as the temperature rises. The degradation is especially significant for conventional Si-based solar cells, since their temperature coefficient is usually between −0.48 and −0.50%/° C. Although GaAs and other III-V semiconductor-based solar cells perform much better at elevated temperatures, the higher manufacturing cost makes them less desirable.
Another issue with the concentration of sunlight is the current crowding effect. In a solar cell, the current is first generated by light absorbed in the solar cell structure, and then collected by the metal grid on the solar cell surface. The concentration of sunlight causes current crowding in the metal grids, where the current increases almost linearly with the concentration ratio. Current crowding can increase the series resistance of the solar cell. Consequently, as the current increases due to light concentration, the solar cell efficiency decreases because of the increased resistive loss.
Moreover, in conventional solar cells, the front metal grids are manufactured using printed silver paste. To minimize shading, the grids are narrow in width. The height of the screen-printed silver grid is typically limited to no more than 30 microns, and the shape of the cross section is triangular. In addition, the resistivity of silver paste after firing can be five to ten times higher than that of the pure silver, due to additives (such as glass grit or adhesives) in the paste. These factors constrain the series resistance of the metal grid, and negatively impact the solar cell efficiency.